Over time, porcelain has gained universal acceptance as a material for dental restorations. Its color approximates natural shading, it may be contoured to form a prescribed occlusal surface, it is durable in saliva, it has reduced thermal conductivity, and it resists abrasion from food and brushing. However, porcelain has a tendency to fracture due to its crystalline structure.
The ability of porcelain to resist fractures has been greatly enhanced by chemically bonding or fusing the porcelain to a metal substrate with the assistance of a suitable interface composition. Although the metal adds strength, it is the chemical bond that inhibits fracture of the porcelain and significantly enhances its strength and suitability in dental applications.
The prior art includes, in the more distant past, a porcelain jacket crown. In its final form it does not utilize a metal foil substrate. However, when constructing a porcelain jacket crown, a metal foil substrate, such as platinum, provided a support post to which porcelain was applied. Platinum was used because it did not adhere to the porcelain and could be easily removed when the crown was complete and ready for use. Such a crown was susceptible to fractures.
The next type of metal-porcelain restoration utilized a cast metal substrate that was typically 0.2 to 0.3 mm. thick and comprised of gold and non-gold alloys. Porcelain was bonded to the cast metal by baking, with the assistance of a suitable interface composition, and the combined metal-porcelain restoration was then fitted to the patient's mouth. The casting process was costly because of the necessary precious materials and time required, and it demanded a high degree of skill in casting techniques. In addition, the fit was often less than desireable and an unsightly metal margin was readily apparent.
In the more recent past, metal-porcelain restorations have been made with a swaged metal foil substrate. These restorations eliminated the need for metal castings and their attendant disadvantages, and permitted one to easily and quickly learn the techniques of swaging the foil around a die. This expedited the manufacturing process, and eliminated the use of gold alloys and time-consuming castings, thereby reducing costs. In addition, the reduction in metal thickness to approximately 25 microns allowed for less reduction of the tooth base and permitted the use of porcelains that were more suitable than those used with cast metal.
However, as with cast metal substrates, the porcelain was not bonded directly to the metal foil. Instead, a series of intermediate steps was undertaken to create an interface between the metal foil and porcelain. These intermediate steps required that a thin layer of tin be electrodeposited on the foil substrate. The tin coated substrate was then baked to oxidize the tin and provide a suitable substrate for bonding. Thus, the porcelain was not bonded directly to the swaged metal foil substrate. Absent the tin-oxide coating, the porcelain would not bond to the substrate.
In addition to the aforementioned steps and materials necessary to create the tin-oxide interface, this prior art interface coating is excessively dark, necessitating the application and baking of one or more opaque porcelain layers so that the restoration would be aesthetically acceptable. Once these additional steps were completed, body or dentin porcelain was applied over the opaque layers, followed by enamel outer porcelain. Thus, the tin coating and subsequent oxidation required additional fabrication time and equipment, subsequent color correction, and additional baking steps for both oxidation and applying an opaque porcelain underlayer.
The first foil substrate metal-porcelain restorations used two platinum foils. One foil was a permanent part of the restoration and the second foil was used only during fabrication. The one foil was tin plated, oxidized, coated with an opaque layer, baked, and eventually bonded to the porcelain so as to extend close to but not beyond the margin of the tooth. The second foil was sandwiched between the first foil and the die or tooth base and was not tin plated. It extended below the margin of the tooth and served as a removeable matrix for the marginal region where there was no inner tin oxide coated foil. When removed, it provided a space for adhesive. The porcelain bonded to the tin-oxide coated one foil but not to the uncoated second foil. Once the bonding process was complete, the inner (second) foil was removed and the restoration was fixed in a patient's mouth. This process resulted in a restoration with a metal-porcelain crown portion and a margin of pure porcelain without a metal substrate. This eliminated an unsightly metallic margin, but the problems associated with the dark tin oxide layer and unnatural appearance persisted. In addition, because of the removal of the foil matrix at the margin, twin foil restorations suffered from two additional problems. One concerned the form that the porcelain would take when it abutted to a foil that it did not adhere to, and the second concerned an increased frequency of cement washout at the margin of the restoration.
Metal-porcelain restorations having single layer foil substrates were subsequently developed, but they still required tin-plating, oxidation, the application of opaque material, and additional baking, along with the extra time, materials, and skills required to complete these steps. Moreover, these restorations still suffered from all of the aesthetic drawbacks associated with the dark tin-oxide layer underneath the dental porcelain.